As our population ages, more people develop bone and/or spinal problems. Metabolic bone diseases such as osteoporosis, osteomalacia, and Paget's disease are usually associated with osteoporotic or soft skeleton, especially in elderly patients.
In particular, osteoporosis, a progressive metabolic bone disease, affects 10 million Americans. For example, approximately 30% of postmenopausal white women in the United States have osteoporosis, and 16% have osteoporosis of the lumbar spine in particular.
Initially, osteoporosis primarily involves the inner layer of bone, the cancellous portion. In the early stages of osteoporosis, the number and strength of trabeculae diminish, weakening the structural stability of the bone. In later stages, osteoporosis will weaken the cortical portion of the bone also. Endocrine, metabolic, genetic, and nutritional disorders further contribute to the development of osteoporosis. Related risks include long-term corticosteroid use, chemotherapy, radiation therapy, age and gender, genetics, and unhealthy lifestyle choices. Similar devastation to vertebral structures can be caused by metastatic spinal disease. For example, other processes, such as cancer, can alter the normal architecture of cancellous bone. By invading and replacing bone marrow, cancers can weaken individual bones in the same manner that osteoporosis does. Surgery is often presented as an option to treat such patients by providing support and/or stability to the patient's bones. While such procedures are often successful, as patients live longer, their risk of osteoporosis increases. Consequently, the presence of osteoporotic bone is an increasingly common challenge to orthopedic surgeons.
In spinal surgery, for example, screws are used in fixation procedures to treat spinal instability by positioning supporting instrumentation along the spine. The use of spinal instrumentation in osteoporotic patients is often complicated by this problem of hardware pull-out or loosening. This complication may occur during surgery, while manipulating the instrumentation, or at any time after surgery. This is a special concern since screws that are inserted in to osteoporotic vertebrae will compress the cancellous bone within the vertebrae whenever the spine is loaded. The cancellous bone will continually be compressed until the screw contacts the endplates of the vertebrae or until enough bone is compressed and a stable column is formed against the screw that prevents its further displacement.
Often, screw failure in osteoporotic bone involves compaction of enclosing cancellous bone during loading; unlike failure in dense bone, which entails screw bending without bone failure. If screw loosening occurs late after surgery, the patient may need either revision of the instrumentation or supplementation with an anterior fusion incurring complications and high costs. Therefore, selection of appropriate instrumentation and a means to augment it is crucial in osteoporotic patients.
Therefore, there is a need for improved fixation assemblies and methods of using the same.